Explosive vaporization in fused silica initiated by a tunable infrared laser

We present evidence for explosive vaporization in fused silica, initiated by picosecond pulses from a tunable free-electron laser (FEL) operating in the wavelength region from 2 to 10 mm. The unusual pulse structure of the FEL, which produces 1 ps micropulses at intervals of 350 ps in a macropulse lasting up to 4 ms, makes it possible to test separately the effects of intensity and ¯uence. We show that thermal descriptions of the ablation process fail in the regime where there is high vibrational excitation density in the solid due to resonant absorption of mid-infrared laser light. # 2000 Elsevier Science B.V. All rights reserved.